Catalytic cracking processes utilizing zeolite-containing catalyst compositions are employed to produce gasoline and light distillate fractions from heavier hydrocarbon feed stocks. Deterioration occurs in the cracking ability of the catalyst which is attributable to the deposition on the catalyst of metals introduced into the cracking zone with the feed stock. The deposition of these metals such as nickel and vanadium results in a decrease in production of the gasoline fraction. Additionally, an effect of these contaminant metals when deposited on the cracking catalyst is to increase coke production and cracking depth as demonstrated by an increase in hydrogen production.
The cracking catalysts to which the method of this invention are applicable are those zeolite-containing catalysts employed in the cracking of hydrocarbons boiling substantially above 600.degree. F. (316.degree. C.) for the production of motor fuel blending components and light distillates. These catalysts generally comprise a matrix which is silica or silica-alumina in association with zeolitic materials. The zeolitic materials employed can be natural occurring or synthetic and which have been ion exchanged utilizing conventional ion exchange methods with suitable cations such as the rare earths so as to improve the activity of the catalyst.
Examples of cracking catalysts to which the method of this invention is applicable include those obtained by admixing an inorganic oxide gel with an aluminosilicate composition which is strongly acidic in character as a result of treatment with a fluid medium containing at least one rare earth metal cation and a hydrogen ion or one capable of conversion to the hydrogen ion.
Petroleum charge stocks to gasoline-producing catalytic cracking processes contain metals which are generally in an organometallo form, such as in a porphyrin or naphthenate with such metals tending to be deposited in a relatively non-volatile form onto the catalyst. Those metals contained as contaminants in such petroleum charge stocks include nickel, vanadium, copper, chromium, and iron and normally comprise less than 1.5 parts per million (ppm) nickel equivalents (ppm nickel + 0.2 ppm vanadium) as metal contaminants. In continuous cracking processes when the accumulation of such metal contaminants onto the catalyst reaches approximately 1,500 ppm nickel equivalents, it is normally necessary that the catalyst be replaced to prevent loss of gasoline production and to prevent increased cracking depth as measured by an increase in hydrogen production.